Field
The disclosure of this application relates generally to retinal imaging, and more in particular to a method for robust eye tracking and an apparatus therefor.
Related Art
Retinal imaging includes various imaging techniques by which two-dimensional (2D) or three-dimensional (3D) images of the back part of the eye (fundus), which includes the retina, optic disc, choroid, and blood vessels, are obtained. Retinal imaging techniques include, among others, fundus photography, Optical Coherence Tomography (OCT), Scanning Laser Ophthalmoscopy (SLO), and combinations of such techniques which are referred to as multi-modality retinal imaging techniques.
The principle of Optical Coherence Tomography (OCT) is the estimation of the depth at which a specific reflection or backscatter of light originated by measuring its time of flight through interferometry. In retinal imaging, backscatters are typically caused by differences in refractive index in transitions from one tissue layer to another. The backscatter from deeper tissues can be differentiated from backscatter originating at more superficial tissues because it takes longer for the light from the deeper tissues to arrive at the sensor. As the retinal thickness is between 300-500 microns (μm), the differences in time of flight are very small, but these differences can be measured through interferometry, by applying specialized image processing algorithms. This permits OCT to obtain 2D and 3D images of the retina. Therefore, most retinal layers can be imaged by well known OCT imaging techniques. However, imaging of specific fundus regions, such as the capillary choroid and foveal cones and rods, although available in research laboratory settings, cannot yet be done efficiently and accurately with commercially available devices.
A Scanning Laser Ophthalmoscope (SLO) is a type of confocal microscope which is optimized for imaging specific regions of the fundus of the eye, such as the capillary choroid and foveal cones and rods. SLO imaging is a technique in which image intensities represent the amount of reflected single wavelength laser light obtained in a time sequence. SLO imaging utilizes horizontal and vertical scanning mirrors to scan a specific region of the retina and create raster images viewable on a display monitor. A fundus image is created by scanning a laser beam over the retina of a subject's eye in a raster pattern, and detecting light reflected from each point scanned to electronically produce a digital image. Beam deflection is achieved by a combination of two light-deflecting scanners including one slow vertical scanner (also referred to as “Y scanner”) and one fast horizontal scanner (also referred to as “X scanner”). Galvanometer scanners or resonant-type scanners, or even acousto-optic deflectors, are typically used for beam deflection. An appropriate optical detector, such as a photomultiplier tube (PMT) or an avalanche photodiode (APD), is typically used to detect the scanning signal.
In SLO imaging, multiple images are taken in sequence for averaging and constructing a panoramic composite image to analyze the status of an eye. For constructing such panoramic images, each frame in the sequence should be obtained at the exact same location of the eye. But it is very difficult to maintain each frame at the exact same position because an eye naturally moves continuously during imaging. In particular, in small Field Of View (FOV) imaging systems such as an Adaptive Optics SLO (AOSLO), where eye movement is quite large compared with the frame size, the imaging area tends go out of the frame easily due to eye movement. Similarly, in wide FOV imaging systems such as a WFSLO, large eye movements such as a blink tend to prevent accurate imaging even if the imaging area remains within the frame.
To maintain the imaging area within the scanning frame and to correct for involuntary eye movement, SLO tracking techniques have been developed. See, for example patent application publication US 2015/0077706, which discloses an ophthalmoscope including a wide field SLO and a small field SLO for stabilizing the small field SLO against a movement of the eye by use of a tracking mirror controlled by both the small field SLO and the wide field SLO. In addition, patent application publication US 2013/0215386 discloses an ophthalmologic apparatus including a WFSLO apparatus and an AOSLO apparatus where a WFSLO beam and an AOSLO beam enter and scan the fundus simultaneously, so that a stable and high-quality AOSLO image is acquired while the WFSLO apparatus confirms which area of the fundus is being acquired. In these techniques, eye position is detected by a position detection apparatus, and the imaging area is shifted by tracking mirrors according to the eye movement.
The existing technologies of image-based eye tracking for scanning laser ophthalmoscopy are vulnerable to the issue of ‘frame out’ due to eye drift, where the tracking system stops working or makes incorrect decisions when the eye drifts out of a mapping range of a reference image. To avoid this large motion, a wide field of view retinal imaging system (the WFSLO apparatus) has been proposed to be used for position detection, as mentioned in the above patent application documents.
However, the existing techniques of AOSLO image-based tracking or WFSLO image-based tracking are not stable enough because WFSLO and AOSLO tracking systems typically work independently. Specifically, AOSLO image-based tracking systems can detect only small movement, but it cannot detect large eye motion. On the other hand, WFSLO image-based tracking systems can detect large eye motion, but the position resolution is much lower than an AOSLO tracking system, thus medium sized movements (those larger than the AOSLO resolution but smaller than the WFSLO resolution) may not be detected with enough precision or may not be detected at all.
Therefore, there is a need for improved eye tracking techniques in SLO imaging.